As this kind of combustion apparatus, there is known one comprising: an upstream-side air supply chamber and a downstream-side mixing passage interposed between a combustion fan and a burner; a zero governor which is interposed in a gas supply passage for supplying fuel gas to the burner so that a secondary gas pressure is adjusted to a pressure equivalent to an internal pressure inside the air supply chamber. The mixing passage has disposed therein a narrowed part in order to accelerate a flow speed of the primary air, thereby making the internal pressure lower than the internal pressure in the air supply chamber. The narrowed part has communicated thereto a gas outlet port at a downstream end of the gas supply passage (see, for example, Patent Document 1). In this arrangement, a differential pressure between an internal pressure in the air supply chamber and an internal pressure in the narrowed part varies in proportion to the amount of supply of the primary air from the combustion fan. Then, the amount of supply of the fuel gas from the gas outlet port which is in communication with the narrowed part varies in proportion to the differential pressure between the secondary gas pressure that is equivalent to the internal pressure in the air supply chamber and the internal pressure in the narrowed part, that is, varies in proportion to the amount of supply of the primary air from the combustion fan. Therefore, by controlling the number of revolution of the combustion fan according to the required combustion amount, the primary air and the fuel gas will naturally be supplied to the burner in an amount according to the required combustion amount.
However, once the number of revolution of the fan has fallen below a lower-limit number of revolution at which the proportional characteristics of the air supply amount can be maintained, the primary air and the fuel gas according to the required combustion amount can no longer be supplied. As a solution, an arrangement was also conventionally considered to dispose an air adjusting valve which varies the opening degree of an air inlet port at an upstream end of the mixing passage such that, in a region in which the required combustion amount falls below a predetermined value which corresponds to the lower-limit value of the revolution of the fan, in a state in which the number of revolution of the fan is maintained at the lower-limit number of revolution, the opening degree of the air inlet port is adjusted by the air adjusting valve, whereby the primary air and the fuel gas can be supplied to the burner in an amount corresponding to the required combustion amount below the predetermined value.
Further, there is also known a combustion apparatus in which a proportional valve is interposed in a gas supply passage. In this kind of combustion apparatus of proportional valve system, a mixing passage is disposed between a combustion fan and a burner or on an upstream side of the combustion fan, and this mixing passage is communicated with a gas outlet port which is on a downstream end of the gas supply passage. In this arrangement, too, once the number of revolution of the fan has fallen below the lower-limit number of revolution at which the proportional characteristics of the air supply amount can be maintained, the primary air according to the required combustion amount can no longer be supplied. As a solution, in a manner similar to the above-mentioned zero governor system of combustion apparatus, there must be disposed an air adjusting valve which varies the opening degree of the air inlet port at an upstream end of the mixing passage such that, in a region in which the required combustion amount falls below a predetermined value corresponding to the lower-limit value of the revolution of the fan, in a state in which the number of revolution of the fan is maintained at the lower-limit number of revolution, the opening degree of the air inlet port is adjusted by the air adjusting valve, whereby the primary air can be supplied in an amount corresponding to the required combustion amount below the predetermined value.
However, if the opening degree of the air inlet port is throttled beyond a certain limit, the amount of the primary air may fluctuate due to minute displacements of the air adjusting valve, resulting in lack of stability in the amount of the primary air. Therefore, the opening degree of the air inlet port will have to be adjusted within a range above a predetermined lower-limit opening degree at which the amount of the primary air becomes stable. Accordingly, the lower limit of the combustion amount will also become a value corresponding to this lower-limit opening degree and, as a result, the turndown ratio cannot be made so large.